1. Field of the Invention
The present invention relates to a method for controlling a time division duplexing (TDD) Tx/Rx switching timing in a cloud radio access network (CRAN), and more particularly, to a method for controlling a TDD Tx/Rx switching timing in the CRAN that can finely control a switching timing between transmission and reception of TDD signals without an additional component added to a digital unit (DU) and a radio unit (RU).
2. Discussion of Related Art
Recently, a centralized/cloud radio access network (CRAN) structure has been widely introduced into base station systems. In the CRAN structure, a digital unit (DU) and a radio unit (RU) of a base station system are separately implemented in order to reduce capital expenditure (CAPEX) and operational expenditure (OPEX) and also secure efficiency in the development of an apparatus.
In the CRAN, typically, DUs are mainly positioned in a DU center provided in a telecommunication office while RUs are installed in a service area that is spaced apart from the DU center. For high-speed transmission and reception of baseband I/Q signals, DUs and RUs are physically connected by an optical link or unshielded twisted pair (UTP) cable.
At present, one of the standards that are most widely used to transmit and receive I/Q data between the DU and the RU is a common public radio interface (CPRI). Further, version 6.0 of the standard may support a maximum line bit rate of 10,137.6 Mbps.
Standards similar to the CPRI include an open baseband remote radiohead interface (OBSAI) defined prior to the CPRI and an open radio interface (ORI) that is a lower standard. For convenience, the CPRI standard will be described below as an example. However, it is natural that OBSAI and ORI are not excluded.
FIGS. 1a to 1d show several connection forms of the CRAN that may be supported by the CPRI specification. FIG. 1a shows the simplest configuration in which one radio equipment control (REC) corresponding to the above-described DU and one radio equipment (RE) corresponding to the above-described RU are connected by a single CPRI link. FIG. 1b shows a configuration in which one REC and one RE are connected by a plurality of CPRI links in order to enhance performance. FIG. 1c shows a star topology configuration in which one REC and one RE are connected by one or more CPRI links. FIG. 1d shows a chain topology configuration in which one REC is connected to one RE by one or more CPRI links and the other RE is connected to the RE by one or more CPRI links. According to the CPRI standard, various configurations such as a tree topology or ring topology other than a described-above configuration may be used to connect the REC and the RE.
In WiMAX and E-UTRA time division duplexing (TDD) environments, a switching timing between transmission and reception of the downlink and uplink signals should be finely controlled. When the DU and RU are installed at the same place, the TDD Tx/Rx switching timing may be controlled relatively easily.
However, in the CRAN in which the DU and RU are spaced apart from each other and a plurality of RUs are connected with one DU by cables having different lengths, it is difficult to finely control the TDD Tx/Rx switching timing due to all the circumstances (for example, where link delays between the DU and respective RUs are different). Of course, the TDD Tx/Rx switching timing may be controlled using each GPS device provided in the DU and all of the RUs. In this case, it is difficult to satisfy a criterion of up to ±16.276 ns in a multi-hop environment because the production cost increases significantly and also an error of tens of nanoseconds or more is inevitable.